Effect of In Situ Animal-Fluid Interactions on Transport and Mixing

Traditional studies of animal-fluid interactions have led to the understanding of factors that affect the distribution, ecology and energetics of swimming organisms. These interactions are commonly investigated by using quantitative flow measurement techniques, which include digital particle image velocimetry. Due to limitations in quantitative flow measurements in the natural environment, animal measurements are conducted in laboratories. Laboratory measurement techniques have been shown to have an altering impact on animal behavior and resulting flow fields. Hence, it is reasonable to question conclusions made about the impact of background flows in the natural environment from measurements conducted in the laboratory. Therefore, an apparatus that will enable the quantitative measurement of flows surrounding a swimming animal in the field is needed to accurately address the effect of background flows on animal swimming and fluid transport. We describe the development of a self-contained underwater velocimetry apparatus that achieves the goal of real-time, quantitative field measurements of aquatic animal-fluid interactions. Using this apparatus, we obtain measurements of flow fields surrounding animals in the field and analyze the effect of background flows on swimming animals. Using a dynamical systems technique called Lagrangian coherent structures to quantitatively compare laboratory and field-generated flows, we find that background flow structures alter fluid transport by swimming jellyfish. From these studies, we define a biologically-relevant metric for animal feeding that is based entirely on the volume of fluid that interacts with the swimming animal. The ability to quantify background flows and their influence on animal-fluid interactions will allow us to broaden our concept of animal-fluid interactions to include the effects swimming animals have on their surrounding environment. This represents a paradigm shift in the analysis of animal-fluid interactions. Recent studies have provided heated debate about whether biologically-generated (or biogenic) mixing can have an impact in the ocean. Arguments for biogenic mixing lacked an efficient mechanism for fluid transport in viscous and stratified flow environments. We present an effective mechanism for biogenic mixing called drift, which is active during swimming, and results in permanent displacement of fluid in the direction of the animal's motion (in unstratified flow). We show that unlike mechanisms that rely on turbulent mixing generated by wake structures, drift is enhanced as viscous effects are increased. While drift has been observed in jellyfish and copepods, to understand its relevance in the global ocean, the effects of stratification need to be considered. By conducting simulations of moving bodies in stratified flow, we show that at buoyancy frequencies on the order of the mean ocean, fluid transport due to drift remains a powerful mechanism through which swimming animals may provide a significant contribution to mixing in the oceans.